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Agronomy Journal 92:173-181 (2000)
© 2000 American Society of Agronomy

WEED MANAGEMENT

Interference between Spring Cereals and Kochia scoparia Related to Environment and Photosynthetic Pathways

Albert J. Fischera, Calvin G. Messersmithb, John D. Nalewajab and Murray E. Duysenb

a Vegetable Crops Dep., Univ. of California, Davis, CA 95616 USA
b Dep. of Plant Sciences, N. Dakota State Univ., Fargo, ND 58105-5051 USA

ajfischer{at}ucdavis.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Kochia [Kochia scoparia (L.) Schrad.; syn. Bassia scoparia (L.) A.J. Scott] is a weed that infests cereal crops in the Great Plains of the USA, often severely reducing yields. Herbicides have controlled kochia, but recently kochia has developed resistance to many herbicides. Nonherbicide alternatives are therefore needed for the integrated management of kochia. Greenhouse and growth chamber competition studies were conducted between kochia, a C4 weed, and barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) to determine the environmental conditions that would render kochia most vulnerable to competition by a small-grain crop. Replacement-series experiments between kochia and wheat or barley were conducted under various temperature, soil moisture, and light conditions. Unlike wheat, kochia growth and photosynthesis were suppressed under cool temperatures. Barley suppressed kochia more than wheat did because of its larger canopy, despite its lower photosynthetic rates. Under high radiation conditions and warm temperatures, growth and photosynthesis were greater for kochia than wheat. Warm temperatures also increased dark respiration and reduced water use efficiency under low radiation conditions, however, thus limiting kochia's competitiveness under a closed canopy. Water stress did not affect competition, although net photosynthetic rates of kochia were greater at photosynthetically active radiation (PAR) values > 400 µmol m-2 s-1. Growth and CO2 exchange rates varied among four different kochia accessions, but growth of all accessions was reduced by shade. Results suggest that a leafy, cold-tolerant crop or cultivar, grown early in the season to produce necessary ground cover, should provide opportunity to suppress kochia.

Abbreviations: DAE, days after emergence • PAR, photosynthetically active radiation • RCC, relative crowding coefficient • RYT, relative yield total(s)


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
KOCHIA, native to Asia, was introduced to the USA from Europe (Phillips and Launchbaugh, 1958) and has become an abundant weed of crops in the Great Plains. Kochia can be very competitive with wheat, sugarbeet (Beta vulgaris L.), and sunflower (Helianthus annuus L.) (Weatherspoon and Schweitzer, 1969; Dahl et al., 1982; Durgan et al., 1990). Kochia has developed resistance to several herbicides (Bell et al., 1972b; Primiani et al., 1990), and further exposure to new herbicides may result in plants resistant to more than one herbicide class (Powles and Preston, 1995). Thus, integrated management of herbicide-resistant kochia should include nonchemical options. One such option is the use of competitive crops (Richards, 1989; Fischer et al., 1997) under conditions that maximize crop interference with kochia.

Differences in the photosynthetic mechanisms of C3 and C4 plants have led to generalizations about the greater competitive ability of C4 than C3 species (Black et al., 1969). These generalizations have been challenged on the basis that environmental parameters such as temperature, soil moisture, and available light affect carbon uptake by C3 and C4 species differently. Thus, growth and competitive success of C3 and C4 plants will ultimately depend on specific growing conditions (Ehleringer, 1978). Therefore, interference (defined here as the negative effects of one plant upon another through competition, allelopathy, or other detrimental processes) between kochia and C3 crops may be affected by environment, and understanding such effects can help identify management opportunities within target environments.

Kochia is a species with considerable morphological diversity, and accessions can also differ in their flowering responses to photoperiod and temperature (Bell et al., 1972a; Thill and Mallory-Smith, 1996). The ecophysiological implications of this diversity are still unclear.

The replacement-series design is used to study the interactions between two species by varying their proportions while maintaining a constant total density (de Wit, 1960). The total density should be high enough for plants to interfere with each other (Harper, 1977), or similar to crop densities found on commercial farms, because these may represent the carrying capacity of growing sites (Spitters, 1980). Replacement-series experiments have been used to study the effect of various growing conditions on the outcome of competition between two species (Patterson and Highsmith, 1989; and Wall, 1993). Here, we define competition as a form of interference in which the acquisition of limited resources by one species reduces its availability to a neighboring plant, thereby reducing its growth.

Our objective was to determine how interference between kochia and spring grains varies under environmental conditions that occur in North Dakota. A mechanistic understanding of such responses was attempted by relating competitive effects to gas exchange rates and photosynthetic pathways. Management hypotheses were formulated based on the observed responses.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
Replacement-series experiments were conducted in the growth chamber or greenhouse to study interference between kochia and wheat or barley. The first was a preliminary study to evaluate growth and interference under common growth chamber conditions. The second was conducted to replicate the first, but with additional fertilizer. Two other experiments evaluated interference under cool, early-season conditions vs. warm, midseason temperatures. Also, two replacement-series experiments were conducted to evaluate interference under conditions of abundant vs. limited soil moisture.

Five additional experiments were conducted in the growth chamber using individually grown kochia, wheat, and barley plants. These experiments were conducted to establish possible correspondence between CO2 exchange rates under varying environmental conditions and interference responses observed under similar conditions in the replacement-series experiments.

Interference Experiments
General Procedure for All Replacement-Series Experiments
Plants of a locally collected accession of kochia and either wheat cv. Grandin or barley cv. Foster were grown in pots containing 2.2 kg of an Arveson sandy loam soil (coarse-loamy, mixed, superactive, frigid Typic Calciaquolls). A total density of four plants per pot was established, which corresponds to about 2400000 crop plants ha-1, within the range of plant populations recommended locally for wheat. Plants were grown for 49 DAE (days after emergence) and each pot received a total of 0.87 g N, 0.76 g P, and 0.72 g K, regularly distributed throughout the growth period. Plants were grown in growth chambers or in a greenhouse. In the greenhouse, sunlight was supplemented with 450 µmol m-2 s-1 PAR from 400 W m-2 high-pressure sodium lamps. Growth chambers were equipped with fluorescent and incandescent light bulbs. The position of pots in the greenhouse or growth chamber was randomized weekly, and unstressed plants were subirrigated daily to near field capacity.

Interference of kochia vs. wheat or barley was studied using a replacement-series approach (de Wit, 1960). Treatments in each replacement series consisted of five crop–kochia mixtures in ratios of 100/0, 75/25, 50/50, 25/75, and 0/100. The actual plant numbers for the 75/25 crop/kochia represent a weedy crop in North Dakota, according to a survey by Dexter et al. (1981). Aboveground biomass (dried at 65°C for 72 h), height, and leaf area were recorded at the end of each experiment.

Growth in mixtures was expressed as relative yield (RYa = biomass per pot of species a in mixture as a percentage of the biomass per pot of species a in monoculture). For a given mixture, the relative yield totals (RYT) were calculated as: RYT = RYa + RYb (Harper, 1977); RYT = 100 indicates that the species are competing for the same resources; RYT > 100 indicates that more biomass is produced by mixtures than monocultures, suggesting that the species are making demands on different resources, avoiding competition or maintaining a symbiotic relationship (Harper, 1977). Competition avoidance can result when the species make different demands on the environment, or when resource acquisition is separated in space or time; this is also known as niche differentiation (Spitters, 1983). Replacement diagrams (de Wit, 1960) were constructed by plotting RY means ± 1 SE against the proportion of kochia in the mixtures. Linear, quadratic, or cubic regression models were fitted to the data; the choice of model was based on comparisons of mean square errors and R2 values.

Relative crowding coefficients, calculated as RCC a vs. b = [(mean yield per plant of species a in mixture / mean yield per plant of species b in mixture) / (mean yield per plant of species a in monoculture / mean yield per plant of species b in monoculture)], were calculated with data from each replacement series (Harper, 1977) and averaged over the 75/25, 50/50, and 25/75 ratios to provide a simple expression of competitiveness between kochia, wheat, and barley (Novak et al., 1993). When organisms in mixture compete for limited resources, the species with the greatest RCC is the stronger competitor.

Kochia Interference with Wheat or Barley
Two experiments, each consisting of two replacement series of wheat vs. kochia and barley vs. kochia, were conducted. Exp. 1 was conducted as described above in the section General Procedures for All Replacement-Series Experiments, except that pots contained only 1.4 kg soil and were fertilized with only 0.36 g N, 0.31 g P, and 0.30 g K. Exp. 2 was conducted as described above in the General Procedures section. Both experiments were conducted in a growth chamber initially set at 17/12°C day/night, then changed to 22/18°C at 30 DAE, to approximate Fargo temperatures for May and June seeding.

The initial photoperiod of 15 h was changed to 16 h at 30 DAE. Light intensity was 550 µmol m-2 s-1 PAR. At harvest, plant biomass, height, and leaf area were recorded, and PAR above (Io) and below (I) the canopy was determined for all crop/kochia ratios with an LI-190SA quantum sensor (Li-Cor, Lincoln, NE). For the 25/75, 50/50, and 75/25 ratios, I was determined for each species by first measuring PAR under the canopies of both species (I for both species), then removing kochia and measuring PAR below the barley canopy (I for barley under competition). PAR below kochia canopies (I for kochia under competition) was determined as I for both species - I for barley under competition. Leaves from kochia, wheat, and barley were separated from stems and dried. The leaf weight/leaf area ratio of 20 randomly selected barley or wheat and 40 kochia leaves was used to estimate the leaf area of whole plants.

Analysis of variance for the three interspecific replacement ratios was performed on growth parameters on an individual plant basis, eliminating the influence of proportion on weight and leaf area differences. (In a 25/75 crop–kochia mixture, the single crop plant would always weigh less or have less leaf area than the three kochia plants, even in the absence of competition.) Crops and ratios were considered main effects and species (wheat vs. kochia and barley vs. kochia) were split-plot treatments (Anderson et al., 1996). Relative crowding coefficients (crop vs. kochia) were calculated from each replacement series as described above and subjected to analysis of variance.

Temperature and Kochia Interference with Wheat
Two replacement-series experiments were conducted to study how the interference between kochia and wheat varies under different temperature–daylength regimes. One experiment was conducted under conditions corresponding to an early seeding at a cool site in North Dakota (15/11°C day/night) and the other experiment was conducted under conditions representing a late seeding at a warm site (22/18°C) in North Dakota. These temperatures were changed at 30 DAE to 19/16°C and 23/19°C, respectively. Both temperature regimes were studied simultaneously in similar growth chambers with a light intensity of 600 µmol m-2 s-1 PAR. Photoperiods were set initially to 15 h for the cool regime and 16 h for the warm regime, and were changed 30 d later to 16 and 17 h, respectively. Each experiment was conducted (run) twice; each time, temperature regimes were switched between the growth chambers to minimize chamber effects other than temperature.

For each experiment, individual plant weights in the three interspecific replacement ratios were subjected to analysis of variance; ratio was considered the main effect and species a split-plot treatment. There were no treatment x run interactions for either temperature regime, so data from two consecutive runs were combined. For each experiment, RCC values for wheat and kochia were calculated and subjected to analysis of variance.

Moisture Stress and Kochia Interference with Wheat
This experiment was conducted in the greenhouse at 25/20°C day/night temperatures and a 16-h photoperiod using two replacement series to evaluate interference effects under two moisture levels: (i) pots irrigated to near field capacity, and (ii) plants subjected to moisture stress. Plants in the stress treatment were irrigated to near field capacity for the first 15 DAE, and thereafter soil moisture was allowed to decrease. At the onset of flaccidness and leaf rolling from moisture stress, plants were watered to near field capacity and turgidity was restored. This cycle from full turgidity to moisture stress was repeated four times. The experiment was repeated under similar conditions.

Analysis of variance for the three interspecific replacement ratios was performed on individual plant weight, with moisture regime and ratio considered main effects and species considered split-plot treatments. There were no experiment x treatment interactions for the moisture regimes, so data from two consecutive experiments were combined. Relative crowding coefficients (kochia vs. wheat) were calculated for each moisture regime and subjected to analysis of variance.

Gas Exchange Experiments
Experiments and Growing Conditions
Five experiments were conducted under growing conditions common for spring cereals in North Dakota to characterize the gas exchange rate of wheat, barley, and kochia as affected by (i) temperatures typical of an early May seeding, (ii) typical June temperatures, (iii) typical July temperatures (wheat and kochia only), (iv) moisture stress, and (v) growth under shade. One plant of either kochia, wheat, or barley was grown in pots with 500 g soil. A fertilizer solution was applied at weekly intervals to provide each pot with a total of 0.78 g N, 0.63 g P, and 0.60 g K. Pots were regularly watered to maintain a moist soil surface. In the moisture stress experiment, water was supplied when leaf flaccidness and leaf rolling were noticed. Gas exchange measurements were conducted halfway between irrigation events. The experiments were conducted in controlled environment chambers under fluorescent and incandescent lights. Growing conditions for each experiment are presented in Table 1 . The position of the pots in the chamber was randomized weekly.


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  		Table 1 Parameters for wheat, barley, and kochia growth in five experiments to study their gas exchange characteristics, and the leaf temperatures at which gas exchange was recorded

 
Experiments with One Kochia Accession
Experiments 1 through 4 in Table 1 were conducted with the same kochia accession. Gas exchange rates were recorded at 40 DAE on young, fully developed leaves using a Li-Cor LI-6200 portable photosynthesis system. Plants were moved for gas exchange measurements, which were conducted inside a growth chamber under a 400 W m-2 high-pressure sodium lamp, at five decreasing light intensities: 1048, 617, 357, 217, and 0 µmol m-2 s-1 PAR, obtained by placing opaque shade cloths above the instrument's console that encloses the leaf. A 3-cm layer of distilled water in a glass tray was suspended below the sodium lamp to help maintain constant leaf temperature. Readings were taken after steady-state CO2 exchange rates were observed. Average CO2 concentration within the cuvette was 361 µL L-1, and average relative humidity was 18%. Average leaf temperatures are presented in Table 1. Leaf contours were traced on paper immediately after completing a set of measurements (five PAR levels), and leaf areas were determined using cutouts of the traced leaves and the weight/area ratio of the paper.

Wheat, barley, and kochia were the treatments within completely randomized designs with four replicates. Gas exchange data were interpreted by fitting a rectangular hyperbolic model

(1)
where Y is net photosynthetic rate, X is irradiance (independent variable), a is dark respiration, b is the slope corresponding to the radiation use efficiency, and c is the photosynthetic rate at maximum irradiance (Loomis and Connor, 1992). Photosynthetic water use efficiency was defined as the quotient of CO2 uptake rates and the corresponding transpirational losses (Allen, 1994). A rectangular hyperbola was also used to describe the relationship between photosynthetic water use efficiency (mmol CO2 mol-1 H2O) and PAR.

Experiment with Four Kochia Accessions
In addition to the kochia line used in Exp. 1 through 4, three other locally collected accessions were included in this experiment (Exp. 5, Table 1) to assess whether CO2 fixation rates and adaptation to shade by the accession used in previous studies described above would compare with those of other kochia biotypes collected from different sites. Wheat, barley, and the four kochia accessions were grown in a growth chamber under two light intensities, 550 and 250 µmol m-2 s-1. The low light intensity was obtained by placing a shadecloth above the plants. Carbon exchange rates were recorded 40 DAE at PAR intensities of 1500, 511, 106, and 0 µmol m-2 s-1 under a 400 W m-2 high-pressure sodium lamp, and aboveground biomass of wheat, barley, and kochia were determined.

Treatments were assigned in factorial combinations of two illumination regimes and six genotypes within a completely randomized design with four replicates. Data were subjected to analysis of variance.


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
Interference Experiments
Kochia Interference with Wheat and Barley
Significant (P < 0.01) main effects and interactions involving species ratios indicated the presence of competitive effects (Anderson et al., 1996). Cereals had symptoms of N deficiency in Exp. 1, and grew poorly in comparison to their growth when fertilizer application was greater in Exp. 2 (Table 2) . In Exp. 1, wheat accumulated less biomass than barley with kochia interference (Table 2), and kochia plants had more dry matter than wheat (Table 2). Barley plants never accumulated more biomass than kochia. Kochia grew better in mixtures with wheat than with barley (Fig. 1a and b) , although the species partially avoided competition with each other (RYT > 100) in both mixtures. The RCC values indicate that kochia was less aggressive against barley than wheat in Exp. 1 (Table 2).


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  		Table 2 Growth of individual kochia (K) and crop (barley or wheat) plants under competition, and relative crowding coefficients (RCC) of kochia in two studies where wheat and barley grew in replacement series with kochia. Experiment 1 received 2.4-fold less N–P–K fertilizer than Exp. 2

 


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  		Fig. 1 Relative yields and relative yield totals (RYT) (open triangles) of wheat (solid circles) and kochia (solid triangles), and barley (open circles) and kochia (solid triangles) in replacement series in (a and b) Exp. 1 and (c and d) Exp. 2. Error bars represent ± standard errors of the means

 
When the experiment was repeated in larger pots and with higher fertility (Exp. 2), barley had greater biomass and leaf area than kochia in each mixture (Table 2). Also, barley accumulated more dry matter and leaf area than wheat. Thus, while kochia and wheat avoided competition with each other (RYT > 100), kochia relative yields were reduced more by barley interference than wheat interference (Fig. 1c and d). Kochia was at least three times less aggressive with barley than with wheat (Table 2).

In Exp. 2, kochia plants were on average about two and eight times less leafy than wheat and barley, respectively (P < 0.01) (Table 2). Also, kochia never had a height advantage over wheat or barley (Table 2). Kochia's smaller canopies (Table 2) were therefore at a disadvantage in competing for light, since canopy light interception, Ln (I/Io), increased linearly with leaf area and height . Kochia was somewhat more efficient than the cereals at intercepting light, however, because of its greater (P < 0.05) light extinction coefficient [k = -0.65 x 10-3, -0.36 x 10-3, and -0.33 x 10-3 for kochia, barley, and wheat, respectively, where k = Ln(I/Io) / leaf area] (Brown, 1982).

Temperature and Kochia Interference with Wheat
A significant (P < 0.01) ratio effect and ratio x species interaction indicated the presence of competition effects. Under cool temperature and short day conditions, wheat produced more dry matter than kochia in all competition mixtures (Table 3) ; both species competed for the same limited resources (RYT = 100), and wheat was more competitive than kochia (Fig. 2 ; Table 3). These cool temperatures were typical of an early seeding date in northwestern North Dakota. Warmer temperatures and longer days favored kochia, although its RCC was the same as wheat (Table 3). The high RYT at the warm temperature regime (Fig. 2) suggests that these species were partially avoiding competition, most noticeably at the 25/75 and 50/50 kochia/wheat proportions.


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  		Table 3 Effects of competition on the aboveground biomass of individual kochia (K) and wheat (W) plants, and on the relative crowding coefficients (RCC), in two replacement-series studies conducted under two temperature regimes

 


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  		Fig. 2 Relative yields and relative yield totals (RYT) (open triangles) of wheat (solid circles) and kochia (solid triangles) grown at 15/11 and 22/18°C day/night temperature regimes in replacement-series experiments. Error bars represent ± standard errors of the means

 
Moisture Stress and Kochia Interference with Wheat
Temperature regime in this experiment was 25/20°C day/night and kochia interference was similar to that observed at the somewhat lower temperatures (22/18°C day/night) in the previous experiment (Fig. 2). Relative crowding coefficients of kochia (1.3 stressed, 1.1 unstressed) were not statistically different (P > 0.05). Relative yield totals > 100 suggest partial avoidance of competition.

Gas Exchange Experiments
Effect of Early Season Temperatures
Temperature during early vegetative development (40 DAE) had a greater effect on the rates of net photosynthesis of kochia than wheat or barley (Fig. 3) . Kochia had greater net photosynthetic rates than wheat at the warm 23/19°C day/night midsummer temperatures (Fig. 3a). However, as temperatures decreased to 21/17°C and 15/11°C, kochia net photosynthetic rates were reduced—but were never lower than barley (Fig. 3b and c).



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  		Fig. 3 Leaf net CO2 assimilation rate as affected by photosynthetically active radiation (PAR) for barley (open circles), wheat (solid circles), and kochia (solid triangles) when the growing conditions and leaf temperature when gas exchange was measured were (a) 23/19°C day/night, 31°C; (b) 21/17°C day/night, 26°C; and (c)15/11°C day/night, 16°C

 
Response to Moisture Stress and Warm Temperatures
Wheat and barley net CO2 uptake rates were low under drought stress and high temperature, whereas kochia maintained high net photosynthetic rates at PAR > 400 mmol m-2 s-1 (Fig. 4a) . However, kochia had greater dark respiratory CO2 losses than the cereals, and net photosynthetic rates and water use efficiency were lower than those of wheat and barley at PAR < 400 mmol m-2 s-1 (Fig. 4a and b). The minimum light transmission through the canopy of 40-d-old plants was about 15% of full sunlight or about 250 mmol m-2 s-1 (data not shown); a PAR < 400 mmol m-2 s-1 may occur in the field under a full barley canopy. Verschwele and Niemann (1992) found less than 20% light transmission by 40-d-old wheat canopies.



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  		Fig. 4 (a) Net CO2 assimilation rates and (b) photosynthetic water use efficiency of barley (open circles), wheat (solid circles), and kochia (solid triangles), as affected by levels of photosynthetically active radiation (PAR), when grown under moisture stress at 23/19°C day/night

 
Shade Effects
On average, wheat and barley tolerated the shade regime better than kochia, and barley growth was reduced the least by shade (Table 4) . However, barley had low average net CO2 fixation rates, consistent with the observations for other gas exchange experiments (Fig. 3). These results are consistent with the observation that barley had the largest leaf area (Table 2) and therefore high light interception ability.


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  		Table 4 Aboveground dry matter, growth reduction, and net CO2 fixation rates of 40-d-old kochia acessions, wheat, and barley grown under 250 or 550 µmol m-2 s-1 photosynthetically active radiation (PAR)

 
The four kochia accessions were quite diverse, as indicated by a fivefold difference in dry matter production and a wide range of CO2 uptake rates (Table 4). However, the low light regime resulted in similar growth reductions for all four kochia accessions. Growth under shade increased the photosynthetic response of the four kochia accessions, barley, and wheat equally, by 37%.


   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
Early-season weed interference is most deleterious for the establishment and success of small-grain crops (Zimdahl, 1980). Environmental factors affected leaf CO2 fixation rates, and the interference between kochia and these two cereals. Although wheat and barley had rather similar morphology, barley developed more foliage than wheat under both fertility levels (Exp. 1 and 2). Consequently, kochia was less aggressive with barley than with wheat. The larger leaf area of barley may have compensated for the low photosynthetic rates per unit leaf area compared to wheat, resulting in more biomass accumulation for barley than wheat. Low CO2 uptake rates in barley therefore did not necessarily result in a disadvantage when competing with kochia for light. Wheat and barley had more erect leaves than kochia, and the canopy of kochia had a greater light extinction coefficient than the cereals. Thus, results suggest that, in a tall crop, combining high leaf area with less erect leaves to increase the light extinction coefficient should enhance canopy interference and competition for light with kochia.

Cool temperatures adversely affected kochia's ability to interfere with wheat and barley. Under cool temperatures and the shade of wheat, the higher energy cost of the C4 pathway than the C3 pathway (Hatch, 1970) contributed to reduced net photosynthetic rates and suppressed growth in kochia. Moreover, low temperatures may reduce metabolic transport in the C4 mechanism and cause CO2 fixation rates to fall below those of the C3 species, which are more adapted to cool temperatures (Berry and Björkman, 1980; Pearcy et al., 1981). Wheat photosynthesis was greatest at cool temperatures, which usually enhance the activity of ribulose-1,5-biphosphate carboxylase (Edwards and Walker, 1983).

Single leaf gas exchange response to light intensity corresponded to the expected behavior of C3 and C4 species (Ehleringer and Björkman, 1977; Pearcy et al., 1981). Consistent with their C4 pathway, kochia leaves at high light intensities fixed CO2 at higher rates than wheat under warm midsummer temperatures. Kochia grew better with wheat at higher temperatures, regardless of moisture stress conditions. Under high temperatures and moisture stress, however, kochia interference did not suppress wheat. Shaded kochia leaves suffered from higher respiratory losses and lower water use efficiency than the cereals under warm and dry conditions. This vulnerability to low light undermined the capacity of kochia to suppress wheat under dry midseason conditions, explaining in part why interference never amounted to full competition for limited resources under warm temperatures. Therefore, although CO2 uptake rates and competitiveness could be related, photosynthetic pathways and rapid measurements of photosynthetic rates by single leaves are not always good predictors of competitive success (Pearcy et al., 1981).

Imposing moisture stress reduced the growth of kochia and wheat (data not shown), but did not substantially change the pattern of interference compared to nonstressed plants. Although kochia is known for having a dense root system under field conditions (Phillips and Launchbaugh, 1958), any advantage of an extensive root system would probably be lost when roots are confined within the limited volume of soil in the pots used in our experiment.

Our replacement-series experiments were not intended to elucidate the ultimate outcome of interference between kochia and the cereals. The experiments were designed to determine the general directions of change in plant interference that can be expected from certain modifications in the growing environment. Relative yield totals were often >100, which may relate to differences between pot culture and dense stand interference, but they also suggest reduced interference in mixtures relative to monocultures due to the presence of alternate paths of resource acquisition resulting from the different structures of kochia and the cereals, or from differences in the exploitation of growth factors (Trenbath, 1976). Plants of a given species often grew better when the proportion of the competing species increased than when competing with plants of like structure (Tables 2 and 4). Symbiotic relationships are also a possibility when RYT > 100 (Harper, 1977), but their contribution in these experiments is questionable, given the presence of strong competitive effects (RYT = 100) of kochia with barley and wheat under cool temperatures.

Kochia has considerable morphological diversity (Thill and Mallory-Smith, 1996). Thus, although similar gas exchange and growth responses to shade were observed among four kochia accessions, large differences in biomass accumulation suggested that they could also differ in competitive ability (Wall, 1995). Our replacement-series and gas exchange studies suggest that light interception by a leafy and cold-tolerant crop, grown early in the season to develop an advantage for light capture, could help suppress the establishment and early interference of kochia. But further knowledge on kochia ecotype diversity is needed to establish how broadly the above findings can be generalized.


   ACKNOWLEDGMENTS
 
The authors wish to thank Dr. Jim Lorenzen for kindly lending us his LI-6200, and Dr. Abbas Lafta for his assistance with the gas exchange measurements.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Contribution of the Cooperative State Res., Educ., Ext. Serv. (CREES), USDA, under Agreement No. 93-34297-8354 and 94-34330-00430.

Received for publication January 4, 1999.
   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 




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